1. Field of the Invention
This invention relates to an optical system capable of continuously varying the diameter of a beam spot condensed on a light receiving part, and more particularly to an optical system which is applicable to a scanning optical system for scanning a light beam to scan a photosensitive medium, or an optical system for optically reading information from an information accumulating member.
2. Description of the Prior Art
Generally, in a scanning optical system such as a laser optical system or the like, a beam expander of fixed magnification is used to obtain a desired beam diameter. However, where various scanning optical systems having necessary different beam diameters exist, a single beam expander of fixed magnification cannot be used with all of these scanning optical systems, but beam expanders corresponding to the beam diameters required in the respective optical systems must be manufactured. For example, in case of the experimentation with and examination of recording apparatus using lasers which have been actively developed in the recent years, the specification differs from one recording apparatus to another with respect to, the beam diameter required and therefore, it is necessary to manufacture a number of beam expanders. Therefore, to alleviate such cumbersomeness, an afocal zoom lens has been desired which can continuously vary the diameter of the scanning beam.
Heretofore, 8 mm camera zoom lenses have been known as optical systems which continuously vary the diameter of an incident parallel beam and cause the beam to emerge as a parallel beam. As an example, the zoom lens disclosed in U.S. Pat. No. 3,679,292 comprises four lens groups. In this zoom lens, the magnification changing area includes the equal magnification intermediately thereof for that purpose, and has no diffraction limit performance.
As another example of the afocal zoom lens, there is an optical system having the magnification changing portion of the ultratelephoto lens disclosed in Japanese Patent Publication No. 24295/1974. This magnification changing portion comprises three lens groups and for the purpose of use thereof, the magnification changing area thereof includes the equal magnification intermediately thereof, and has no diffraction limit performance.
As still another example of the afocal zoom lens, there is a long wave infrared ray afocal zoom telescope disclosed in U.S. Pat. No. 3,947,084. This is a zoom lens for use in an infrared ray radar apparatus and video apparatus, and comprises four lens groups.
The above-described afocal zoom lens systems are lens systems in which, for the purpose of use thereof, the magnification changing area includes the equal magnification in the intermediate portion thereof, and each lens group forming the lens system assumes a complicated construction to eliminate the various aberrations characteristic in usual photography of an object.